So often when an article is written with the slightest mention of alien life, the comments section becomes filled with comments such as “Aliens do not exist, period” or “UFO’s are on Earth and there is a huge conspiracy” with almost no in between. But I am here to say that neither of those comments is likely to be correct.

I will give you my thoughts now. The possibility of alien life visiting earth is entirely unlikely, possible, but unlikely. But I can see the desire to believe such things; it's exotic and exciting to imagine another being here with us. But honestly, I might be slightly frightened if the first life we found in the universe was life which has been spying on us for decades without showing themselves at all. Of course they may have some sort of code not to interfere with non-interstellar beings - but still. But realistically, life in the universe is almost 100% certainly going to be found outside of Earth.

Take the Miller-Urey experiments, they reproduce the conditions found on early planets and - given the right chemicals (which happen to be found anywhere you look in the galaxy and beyond) - amino acids are created. For those of you that don't know, amino acids are crucial to life here on Earth and may be equally crucial elsewhere. Study has shown that in early star systems these conditions are likely to be reproduced on young planets.

Another crucial element to life as we know it is liquid water. Amazingly enough, water appears to be very common in our galaxy. Comets, nebulae, star formation regions, and even some extrasolar planets have been found to contain water. The problem is finding it in liquid form. This is where the concept of a "Habitable Zone" comes in. This zone, also known as a goldilocks zone, is the area around a star where it's not too hot and not too cold for liquid water to exist on a planet. In the past week or so we discovered the first exoplanet (Gliese 581g) that is firmly within it's habitable zone and therefore the first planet which is entirely capable of maintaining liquid water on its surface.
To expand on that, within that very star system is yet a second potentially habitable planet as well - Gliese 581d. It is on the cold edge of the star's habitable zone, but with a thick enough atmosphere its greenhouse effect could keep it warm enough to maintain liquid water on its surface. This planet's position is comparable to Mars in our solar system in that they are both on the far edge of the habitable zone, the only difference is that Gliese 581d has enough mass to maintain an atmosphere which can produce that strong greenhouse effect.

Now let's look at some statistics, Gliese 581 is just about the 200th closest star to Earth, and within those 200 stars are 2 planets which are definitely in the habitable zone (Earth and Gliese 581g.) Assuming that Gliese 581g is not a lucky fluke, we can say that 1 in every 200 stars has a planet which can support liquid water on its surface. Apply that ratio to the Milky Way galaxy which has about 200 billion stars, and you’ll find that we have 1 billion planets in the habitable zone in our galaxy alone. There are 100 billion galaxies (at least) in our universe, each of which containing, on average, hundreds of billions of stars. Do the math, we're not alone.

If you want proof rather than statistics, and all of us do, then wait for the results from the Kepler mission which is searching for habitable, Earth-like planets. Furthermore we can wait for astronomers (myself included) to study the atmospheres of these potentially habitable planets to determine whether they have life and even intelligent life.

Anyway, to wrap it up:

Aliens on Earth = unlikely.

Aliens elsewhere in the galaxy = extremely likely.

About the Author: Brennen Proffit is a college student who is majoring in Astronomy and Physics at Sierra College in Northern California. He is extremely passionate about exploring the Universe and beyond, and hopes to be an Astrophysicist working for NASA one day.